THE OREGON TRAIL
Seasoned mountain men Jim Bridger and Louis Vasquez establish Fort Bridger on the Green River to re-supply migrants traveling the Oregon Trail. Theirs is perhaps the first mountain outpost not designed as a trading post for trappers.

The Great Migration, a party of one thousand pioneers, heads west from Independence, Missouri, on the Oregon Trail, guided by Dr. Marcus Whitman, who is returning to his mission on the Columbia River. Forming a train of more than one hundred wagons, and trailing a herd of 5,000 cattle, the pioneers travel along the south bank of the Platte, then cross north to Fort Laramie in Wyoming. Here they follow the North Platte to the Sweetwater, which leads up into South Pass. Once through the pass, they cross the Green River Valley to newly established Fort Bridger, then turn north to Fort Hall on the Snake River, which leads them to Whitman's Mission. Once in Oregon, they strike out along the Columbia for the fertile lands of the Willamette Valley, the endpoint to a journey of 2,000 miles. After the mass exodus of 1843, the migration to Oregon becomes an annual event, with thousands more making the trek every year.

Mormon leader Joseph Smith and his brother, Hyrum, are killed by a mob at Carthage, Illinois. Brigham Young becomes the new head of the church.

James K. Polk is elected President with the slogan "54-40 or Fight" -- a promise to set the disputed northern border of the Oregon Territory at 54 degrees, 40 minutes by diplomacy or war, and an implicit promise to expand American territories in every direction.

TEXAS AND THE MEXICAN WAR
Outgoing President John Tyler signs a congressional joint resolution to annex Texas and make it part of the union. In response, Mexico severs diplomatic relations with the United States. When Texas accepts annexation, newly-elected President James K. Polk sends a force under General Zachary Taylor to the Mexican border.

At the same time, Polk sends a representative to Mexico City to offer financial compensation for the loss of Texas and to explore whether Mexico will sell the territories of California and New Mexico for a combined $40 million. Insulted, the Mexicans reject the American proposals and prepare for war. Texas enters the Union at year's end.

In March, American forces under Zachary Taylor cross the Nueces River, which Mexico regards as the Texas border, and take up positions along the Rio Grande, which is the border Texans claim. In response to this provocation, a brigade of 1,600 Mexicans crosses the river in late April, where they overwhelm an American cavalry patrol and then wait for the main body of the Mexican army to press the attack. When word of this encounter reaches Washington, President Polk takes the opportunity to declare war on Mexico.

By early May, nearly 4,000 Mexican soldiers have converged on Palo Alto, where they surprise Taylor's 3,000 troops on an open field. Bringing his light field artillery to the front, Taylor turns back the Mexican charge, forcing a retreat. The battle is an early example of the carnage to come when industrial age weaponry confronts traditional battlefield tactics. Over the next two years, more than 13,000 Americans die in the Mexican War, which prepares a generation of military leaders for the Civil War.

CALIFORNIA AND THE MEXICAN WAR
In March, John C. Fremont, on his third expedition through the West, raises the American flag over California at an improvised fort near Monterey, but he soon abandons his impetuous efforts and turns toward Oregon. On the way, however, he receives word of the impending Mexican War and returns to California to play a part in its conquest.

In June, Fremont joins forces with a group of Americans who capture Mariano Vallejo, the amicable commandante of the Sonora region, and proclaim California an independent republic. But their "Bear Flag Revolt," named for its distinctive banner, comes to an end in July, when American naval forces arrive in Monterey and take control of the port without firing a shot.

Over the following months, American troops under Commodore Robert F. Stockton, aided by Fremont's so-called California Battalion, capture San Francisco, San Diego and Los Angeles without bloodshed. In Los Angeles, however, the American occupation force stirs up violent resentment, and by October they are driven out by a guerrila force led by Anrés Pico, brother of the departed California governor.

Stockton's first attempt to regain control of Los Angeles is repulsed, and while he regroups, an American force arrives from New Mexico, commanded by General Stephen Kearny. Attacked by Pico's insurgents at San Pascual, Kearny's troops suffer heavy losses, but with Stockton's aid they reach safety in San Diego. Early the next year, Stockton, Kearny and Fremont combine forces to recapture Los Angeles, with Fremont accepting the insurgents' surrender in the Capitulation of Cahuenga on January 13.

THE CALIFORNIA GOLD RUSH
On January 24, James Marshall, a veteran of the Bear Flag Revolt, discovers gold on the American River at Coloma while building a lumber mill for John Sutter. A brief report of the discovery appears in a San Francisco newspaper in mid-March, where it goes mostly unnoticed.

In May, Sam Brannan, a Mormon elder who owns a store near Sutter's Fort, arrives in San Francisco with a bottle of gold dust and a plan to draw potential customers for his supplies. Walking through the streets with the gold dust in his hand, he shouts, "Gold! Gold from the American River!" Brannan's publicity stunt sets off a gold rush that will draw fortune-hunters from around the world.